I have a quarter sized hole broken in my rear view mirror housing, on the front side. Right now covered with duct tape. I’d like to glue a piece of black plastic (cut from a plastic bottle) on it for a cheap fix. I plan to use black latex caulk, outdoor rated, unless someone can suggest a better adhesive?
I like to use a two-part epoxy resin for that kind of thing. They tend to hold better than just about anything that comes premixed. Peruse the glue section at your local hardware store for the best two-part adhesive you can find.
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One advantage of RTV like adhesive is that it will allow for thermal expansion contraction differences between dissimilar materials. Certainly, the forward facing surface of a black mirror is going to experience great swings in temperature.
A plastic bottle is going to be smooth compared to the textured surface of the mirror housing. Also not matching the contour so it may be more noticeable than you envision. There are patch systems out there that have surface features to help match the finish. A guy filled and stamped my dash pad that had a gash in it and you can hardly tell it was ever there.
You could glue in the bottle plastic from the inside (slightly larger, bent and fished inside, then pulled tight using tape or plastic rod you later cut off). Once a backer is in place, you can fill the smaller void with your caulk and use some textured top cover to impart a surface finish. All depends on your desire for how noticeable the patch is.
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25 years ago a bear broke off my side-view mirror (I was hiking). I glued it back with a 2-part epoxy. 6 months later, when the temperature dropped to -10, it fell off; fortunately it was parked in my driveway. I bought an epoxy rated to -40, glued it back on and ran a wire that I anchored in the frame of the vent window (an exact replacement is $120; the generics look cheesy) so I wouldn’t lose it if it fell off when I was driving. It hasn’t budged.
I’d look into the chemical solvent type glues of the type used to glue PCV and CPVC pipe. It also works on ABS plastic and others. Read the label as some are more universal than others. Unlike epoxy it doesn’t glue the 2 pieces together as much as chemically weld the 2. I wouldn’t use RTV. I think it will heat up, dry out and fall off.
I just had to replace a range hood duct that the builder RTV’d to the house when they built it 15 years ago. It appeared they felt the more is better approach was best. I had to use music wire to separate that thing from the house without pulling off the siding. Galvanized duct facing south-east. Heat, weather for 15 years. The RTV was still quite supple- and stuck!
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I’m going to suggest you try and do a more professional repair and use something like JB Weld to fill it in. It sticks to most everything and you can sand it to blend in, then paint to match. I used it to fix my wife’s plastic griddle frame that I dropped and broke a chunk out of. A little sanding, flat black paint and buff a little to match and I can’t even see the repair. And it’s dishwasher safe.
It depends on the RTV. The original silicone RTV never really finished curing, and it would “dry out” as the short chain molecules evaporated. Modern curing systems don’t behave that way, but you can still buy old-style RTV silicone systems. Silicone might be overkill, unless extreme temperature excursions are expected. @RandomTroll offered a good example.
I think the most important part is to prepare the surface properly. Clean both surfaces with a good solvent. Since this will alter the surface anyway, I’d use a 50/50 mix of isopropyl alcohol and acetone. That way both polar and non-polar contaminants are removed. Then roughen the surface to provide good keying for adhesion. This is especially easy if the repair is made inside the mirror body. I rally like that idea, @TwinTurbo. After the repair piece is bonded and cured, you can fill the valley with an adhesive that looks like the mirror body. If you want to match the body texture, wait until the filler gets tacky and won’t flow after you score it.
I bet you never heard of plastic fusion welding. Almost any type of plastic can be welded with this process and it will be a permanent fix. Bodyshops around North America are using this process with great success. http://www.soudureplastiquequebec.com/en/
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I think the latex caulk will work ok, better though would be silicone caulk. Black silicone caulk is available in auto parts stores as gasket sealant. If I had that problem I’d probably just use JB Weld (original) b/c it holds about as good as any glue around, and I have a tube of it on hand, and it is black. The two downsides w/JB is first, that once set it is very difficult to impossible to reverse, and second, you have to let it set for 24 hours before allowing any stress on the repair. To avoid the 24 hour wait, 5 minute epoxy would work well too, but I’m not aware of any version that comes in black that is readily available.
Several good suggestions here I see. I could only add hot glue as a possibility.
My go to glue is shoe goo, it is clear, secure it for 48 hours in 40 degree plus weather, pre clean with denatured alcohol, best yet for me!
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I have used that with excellent results gluing together broken ABS fairing parts for sport bikes. These glues actually dissolve the edges of the plastic parts and weld them together. These cements are also very specific to the materials being glued together. ABS, PVC, CPVC. Trying to glue polycarbonate or HDPE together with these glues is mostly a waste of time.
High density polyethylene or HDPE can be welded by lighting a rod of HDPE on fire like a candle and then letting the HDPE candle drip molten HDPE on the pieces you want to weld together.
On non flexible plastic to plastic,
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I’ve also found that superglue - cyanoacrylate (sp?) does a fine job making 2 plastics stay together.
This is nothing new, it’s been around for awhile now
Why do you seem to assume we’d never heard of that process?
To be brutally honest, it reminds me of a prominent person who learns something new to HIM, then assumes that nobody else knew about it, either 
2 part black plastic epoxy. I used the stuff several times with great success. I even used it on a Jeep Cherokee plastic radiator. I glued the top radiator hose neck back on after it snapped off. My son was trying to remove the radiator hose. It never leaked once.
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Bizarre! no one mentioned this process before I did.
many good suggestions, thanks.
When gluing molded plastic parts, make sure you get rid of the mold release agent that may still be on the plastic.
Release agent is the Pam you spray on your waffle iron before putting the batter in the iron.